Modern radio receivers are typically equipped with a squelch circuit to disable the receiver's audio amplifier after the radio frequency carrier is no longer being received by the radio. The squelch circuit prevents noise from being heard at the receiver's speaker during normal radio operation. The basic purpose of a noise squelch circuit is to detect the change in the discriminator output noise as the received carrier strength changes. As the RF carrier strength increases, the discriminator noise decreases. Because the higher frequency noise falls or "quiets" more rapidly than low or mid-range noise and provides faster response time, it is used in most noise squelch circuits to determine the squelch decision (whether to squelch or unsquelch the receiver).
Typically, part of a radio's squelch circuit includes a: noise amplifier stage; pre-emphasis filter stage; limiter stage; a high pass filter stage; noise squelch rectifier stage; DC amplifier stage; and finally a comparator stage for comparing the received signal with the preset squelch level in the radio which is normally set by the radio user.
In FIG. 1 a prior art high pass filter 102 and squelch rectifier circuit 104 such as those used in prior art squelch circuits is shown. In it's typical mode of operation an output signal will reach the output terminal (OUT) of circuit 100 only when the input signal at the input terminal (IN) reaches a level where it can activate the normally "off biased" transistor. Several problems associated with a rectifier and high pass filter combination as shown in circuit 100 include: the compensation for temperature variations; compensation for component variation due to process variation; and the problem of having to vary the components depending on the bandwidth of operation of the radio utilizing circuit 100. Full wave rectifiers using the same bias off scheme are also available, but they share the same problems. However, two of the advantages of such rectifiers are excellent threshold squelch sensitivity and fast transient response for good channel scan performance. Various other half and full-wave rectifier designs exist employing feedback principles which eliminate some of the problems of off-biased rectifiers but which invariably suffer poorer threshold or transient performance or create new problems of their own. A need therefore exists for a squelch rectifier circuit which can overcome the problems associated with the prior art circuits.